The invention relates to a filter for gases, which is of the type presented in the preamble of the appended claim 1.
Such a gas filter is intended, e.g. when placed in a ventilation duct, to remove gaseous impurities from inlet air of a room.
Harmful gases are commonly removed from incoming air with filters which are placed in the ventilation duct and which contain a granular material having a large specific surface. Gases to be filtered out with such filters include e.g. sulphur dioxide, oxides of nitrogen, chlorine, hydrogen chloride, and ozone.
In addition to ordinary working rooms and lounges, there are a group of special rooms where considerably higher requirements are set for the purity of air. Such special rooms include e.g. so-called clean rooms, where e.g. components for sensitive electronic devices and/or such devices are manufactured. Also the spaces where these devices are used must often be protected even with device-specific filters, to secure their reliable function.
The filtration of incoming air has been and will be increasingly used substantially also in connection with various vehicles and work machines, where particularly the filtration of exhaust gases is important, as well as the filtration of such impurities, which are handled by using the vehicle or work machine (e.g. spreading of pesticides with a tractor).
It is generally known to raise the filtration efficiency of filters by increasing the time of contact or retention between the filter material and the air to be filtered, e.g. by increasing the thickness of the filter, i.e. the thickness of the layer through which the air to be filtered flows. A problem in this case, however, is the increasing pressure loss, which is not advantageous for the operation of the ventilation system.
A filter is known from French patent 7232725 (publication number 2199479) where the flow of gas to be filtered is intended to pass through a filter material layer. Such layers can consist of two fibre material layers and a granular adsorbent placed therebetween. In the actual filter structure, these layers can be placed in parallel to form a stack, or, as shown in FIGS. 8 and 9 of the publication, wound in a spiral form and placed in a housing, wherein the material to be filtered is passed in the axial direction and simultaneously from the core of the spiral outwards.
Furthermore, European patent publication 504389 discloses a filter structure in which the material to be filtered is intended to pass through layers of a filter material wound in a spiral form, from the core outwards. The filter material is produced by blending adsorbent granules with adsorbent fibres.
Also in these filters, when a gas to be purified is passed through the filter material containing an adsorbent, one must consider the flow resistance caused by the material, with the problems presented above.
Finnish patent FI-94724 discloses a method for manufacturing a filter for gases, wherein the filter is composed of superimposed layers made of a carrier material which is corrugated and impregnated with a solution inducing chemisorption, whereby the longitudinal direction of the waves coincides with the direction of passage of air, and the layers make up separate, elongated flow cells. The carrier material is an aluminium sheet coated on both surfaces with alumina which makes a porous surface. Such a filter does not induce flow resistance to the air to be filtered, but a considerably larger contact surface is achieved in the same volume than by using granular materials. A filter with the same basic structural type is also known e.g. from U.S. Pat. No. 4,391,616.
It is an aim of the present invention to present a novel type of filter structure which combines small flow resistance and good filtration efficiency, thanks to the large surface area. For achieving this aim, the filter according to the invention is primarily characterized in what will be presented in the characterizing part of the appended claim 1.
In the filter, the adsorbent filter material constitutes also the walls separating the longitudinal flow channels. In these flow channels, the flow takes place between their starting and terminal ends in the direction of the filter material, i.e. the filter material adsorbs the specified impurities from the passing gas flow. In the filter material layer, which extends primarily in the direction of one plane, there are at least two superimposed layers: a porous support layer and a porous adsorbent material layer. Furthermore, the filter material layers are separated by elevated areas deviating from the main direction of the filter material layer, such as elongated spacers rising from the surface of the material layer which has otherwise an even thickness, or tops of waves in a corrugated material layer with a substantially even thickness, which also contain porous adsorbent material in their inner volume. The filter material layers can be multi-layer structures in such a way that on one side of the support layer there is an adsorbent material layer forming the porous outer surface of the filter material layer, or the filter material layer has preferably a structure where the layer of the adsorbent material is placed between two porous support layers, wherein the support layers constitute on both sides the outer surface of the filter material layer.
The elongated flow channels are thus limited in the directions perpendicular to the flow direction by walls containing an adsorbent: in the directions perpendicular to the filter material layers on opposite sides by the areas between the spacers of the filter material layer or the wave bottoms of a corrugated material layer and a straight material layer, and in the direction of the filter material layers on opposite sides by said spacers or, correspondingly, the wave tops of a corrugated material layer.
In view of other advantageous embodiments, reference is made to the appended dependent claims and the description below.